A fuel cell in which fuel supply spaces each disposed on one side of an electrolyte layer and supplied with a gaseous fuel are connected in series to each other has been put into practical use. Further, a fuel cell in which a plurality of fuel supply spaces are disposed in a cascade system wherein the fuel supply spaces are connected in series such that the number of fuel supply spaces connected in parallel to each other is gradually decreased toward a downstream side has also been put into practical use. By the cascade system, the reduction in flow rate on a downstream side of a gaseous fuel due to consumption of the gaseous fuel through the electrolyte layer can be compensated, and stable supply flow rate of the gaseous fuel in the fuel supply spaces can be secured from the most upstream side to the most downstream side.
An air breathing fuel cell in which a polymer electrolyte membrane is used as an electrolyte layer and one side of the polymer electrolyte membrane is in communication with the atmosphere, and which generates electric power by an electrochemical reaction of a gaseous fuel with the atmospheric oxygen has been put into practical use. Since the polymer electrolyte membrane is not a completely airtight membrane, when the polymer electrolyte membrane is disposed between a fuel supply space and an atmosphere communication space, atmospheric nitrogen will intrude into the fuel supply space from the atmosphere communication space by concentration diffusion. Since the nitrogen that has intruded into the fuel supply space lowers the partial pressure of the gaseous fuel in the fuel supply space to lower the power generation efficiency, it is desirable to purge the impurity gas containing nitrogen in the fuel supply space periodically.
U.S. Pat. No. 6,960,401 discloses a technique of discharging impurity gas by purging.
In a fuel cell disclosed in U.S. Pat. No. 6,960,401, not only the impurity gas but also a considerable amount of hydrogen gas remaining in a fuel supply space on a most downstream side is discharged into the atmosphere. Therefore, devices on which such fuel cell is mounted must be designed on the assumption of the discharge of a fuel gas.
Further, if electric power for maintaining an open/close valve in an open sate is additionally extracted from the fuel cell with so lowered output as to require purging, there may be cases where the operation of the device having the fuel cell mounted thereon does not operate well. Alternatively, the output may be temporarily reduced due to the lowering in fuel pressure during a purge operation.
Moreover, if the output of a fuel cell unit on a most downstream side is reduced by stored impurity gas, the output of the entire fuel cell having a plurality of fuel cell units connected in series will also be reduced due to the affect of the output reduction of the fuel cell unit on the most downstream side. Since the output of fuel cell unit on the most downstream side starts to be reduced in a relatively short period of time accompanying the increase in the partial pressure of the impurity gas, if the output of the entire fuel cell is to be maintained high, purging needs be considerably frequently performed. If purging is frequently performed, the consumption of the gaseous fuel wastefully discharged in the atmosphere increases, and the gaseous fuel in a fuel tank does not last for a long period of time.